Unity custom shapes. Wall connections

Hello,

I'm programming 3D room planner and i have a couple of questions.

I've seen example applications like this: http://www.ihome.pl/zbuduj , http://www.lovemyhome.net/decorator . These applications are done in Unity3d.

My question is: How can i connect walls depending on angle between these walls?

Here you can see two walls. First wall (smallest) is already built meanwhile the second one is in bulding progress:

Second wall built, building third wall. As you can see, two walls were connected by some shape:

The third wall connected to second wall:

And the most demonstrative picture is here:

Beforehand thanks a lot!